Hey there arturdo,
I'm assuming you understand the trig ratios cosine and sine refer to the angular relationship of a right-angled triangle.
Imagine you have a right triangle with a hypotenuse length of 1 unit.
Assume that one corner has angular measure 0 degrees. This means, in reality, the triangle has the appearance of a straight line 1 unit long.
Taking this corner angle of 0 degrees, the cosine ratio can be shown as:
Trust this helps.